1. Field of the Invention
The present invention is directed to a method for selectively etching a semiconductor wafer. More particularly, the present invention is directed to a method for making electrode tips for tunnel current sensing devices in silicon.
2. Background of the Art
The Kenny et al article entitled "Electron Tunnel Sensor Technology", presented at the first national conference and exhibition of NASA's technology for transfer in November of 1990, describes a micromachined servo accelerometer that utilizes a tunnel current sensor. The accelerometer is micromachined from silicon and includes a cantilever spring with an integral electrode tip.
The electrode tips are formed directly from the silicon substrate. An oxide-coated surface is first patterned by photolithography to leave only a 60 um.times.60 um square of oxide for each tip. When etched in EDP, the edges of the oxide are undercut. When the undercutting is complete, the square of oxide is carried away leaving a pyramid-shape silicon tip. The active surface of the tips are prepared by evaporation of a 3000 Angstrom Au film through a shadow mask. A gold film is deposited over the tip to form a tunnel current electrode.
One deficiency associated with the foregoing method is that the wafer must be removed from the etchant after the masking material becomes detached. This deficiency becomes particularly noticeable when multiple electrode tips are created on a single wafer or on multiple wafers. The tips are very susceptible to the etchant and the first tips to be completed may be partially etched away before etching of the remaining electrode tips.